Ductile electrical resistance alloy



July 26, 1960 A. s. DARLING DUCTILE: ELECTRICAL RESISTANCE ALLOY Filed July 29, 1959 .n. ow Emu ma 19mg om o@ 9 o@ om 2 om ow o. A/WIPE u. m l .0:09

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i Alan Sydney Darling,

United States Patent DUCTILE ELECTRICAL RESISTANCE ALLOY Northwood, England, assignor to Johnson, Matthey & Company Limited, London, England, a British company Filed July 29, V1959, ser. No. 330,384 Claims priority, application Great Britain Ang. 6, 1958 1 claims. (c1. '1s-134) This invention relates to an improved alloy and more particularly to a high specific resistance alloy suitable for use in the -fabrication of resistances for high precision electrical apparatus or equipment.

Resistance alloys for use in precision electrical apparatus are required -to possess cer-tain inherent characteristics. They should, for example, have an inherently high specific resistance, and a low temperature coeiiicient of resistance, be resistant to corrosion and abrasion and have adequate tensile strength, whilst at the same time they should be sufficiently ductile to enable them to be worked into wire.

Numerous alloy compositions consisting of base metals only, of base metals alloyed with noble metals, and compositions composed solely of noble metals are known for use as resistance alloys, but none of such alloy compositions as heretofore used or proposed possesses all the desirable characteristics required of a high resistivity alloy for precision electrical equipment. Consequently, manufacturers of high precision electrical equipment have always hitherto been faced with the necessity of compromising and of sacriiicing one or other of the desirable characteristics in order to obtain the others.

For example, base metal resistance alloys, whilst possessing some of the desirable properties, such as high specific resistance and ductility, are susceptible to corrosion or to the formation of high resistivity surface oxide films, Which render them quite unsuitable yfor use as slide wire elements in precision measuring and control equipment.

Many noble metal alloys, on the other hand, whilst being highly corrosion resistant, exhibit a relatively low specific resistance and have poor mechanical properties.

f the noble metal resistance alloys, those based on, or containing, palladium have, in general, higher resistivities than platinum alloys. Examples of palladium alloys, known for use as resistance alloys are the binary palladium-tungsten, palladium-chromium, and palladiumgold alloys.

,f Palladium-tungsten alloys containing up to 20% tungsten, have an inherently high specific resistance, but

'are not suiiiciently ductile to be easily workable.

Palladium-chromium alloys also exhibit a high spel' ciiic resistance, but develop on the surface a thick oxide skin which renders them unsuitable :for use as slidewire material.

The binary palladium-gold alloy, containing 40-60% palladium is known to have a stable resistance. The alloy, however, possesses only a moderately low temperature coefiicient of resistance, has poor mechanical prop` erties and is not resistant to abrasion. The specific resistance of this alloy, moreover, is too low vfor precision instrument purposes.

More recently, a ternary palladium-gold-iron alloy composed of 20-62% Pd, 2870% Au and 4-l8% iron has been proposed for use as a potentiometer wire. This alloy, whilst having the desirable property of high spe- 2 t f' ciiic resistance together with resistanceqtocorrosion and' high tensile strength and ductility, suffers vfrom the disadvantage that carefully controlled heat-t'reatment ofthe alloy is necessary in order to develop the optimum electrical properties.

The principal object of thisinventionfis'to provide an improved resistance alloy for use in precision'electrical apparatus or equipment.

Another object of the invention is to" provide a duc'- tile, stable resistance alloy having highspe'ciiic resistance, high tensile strength and resistance to' corrosion.

A further object of the invention is `to provide a noble metal resistance alloy suitable for use in precision electrical equipment and apparatus, which does not require heat-treatment in order to develop itsoptimum electrical properties.

According to this invention, a ductile stable high resistance alloy is composed of 16-90% palladium, 5-64% gold and 2.5-20% molybdenum, apart from impurities.

-An alloy embodying the invention and which will be found to be eicient in use is composed of 63% palladium, 27% gold and 10% molybdenum, apart from impurities. This alloy has a specific resistance 95.2 microhms per cm.3 as drawn and annealed and a positive temperature coeiiicient of 0.00026 per C.

Another alloy embodying the invention, which has a specific resistance of 77.5 microhms/cm.3 as dra-wn and annealed and a positive temperature coeicient of 0.00013 per C. and is, therefore, slightly less eiiicient than the 63% Pd, 27% Au and 10% Mo alloy, is composed of 45% Pd, 50% Au and 5% Mo apart from impurities.

The single iigure of the accompanying drawing shows graphically the specific lresistance of various alloys in the as-cast condition'emlbodying the invention, the speciic resistance in microhms/cm.3 having been plotted against varying content of gold in weight percent and in respect of molybdenum contents of 5% Mo, 10% Mo and 20% Mo.

lIn a modification of the invention Ythe speciic resistance of my improved alloy may be further increased without detrimentally affecting the workability of the alloy by inclusion of up to 4% of iron in the alloy.

For example, a particularly suitable alloy may be composed of 41.5% Pd, 50% Au, 5% Mo and 3.5% Fe, which will be found to have in the hard drawn condition a specific resistance of 102 microhms per cm.3. The characteristics of this alloy are ygiven in the following table.

.The high degree of stability of this alloy isdemon- Y strated by the fact that, after annealing at' 105 Cffor 1/2 hour, the resistivity of the alloy decreased only to 99.6 microhms per cmi.

My improved alloy will be found to have a specific" resistance within the range of 22-125 microhms per cm.3, to remain'sta-ble over long periods and to possess adequate tensile strength and ductility combined with corrosion resistance, which renders it eminently suitable A 3 for use as resistance material for precision resistance electrical apparatus or equipment. Moreover, the alloy does not require heat-treatment in order to develop its maximum lClriCal Properties.

What If claim is.:

1. A ductile Vstable high resistance alloy composed of 16-90% pal1adium,r56j4-% gold and 2.5-20% molybdenum apart from impurities. A

2. A ductile stable high resistance alloy composed of 63% palladium, 27% gold and 10% molybdenum.

3. A ductile stable high resistance alloy composed of 45% palladium, 5,0% gold and 5% molybdenum.

4. A ductile stable high resistance alloy composed of 16-90% palladium, 5-'6'4% gold, 2 5-20% molybdenum and up to 4% iron, apart from impurities.

5. A ductile stable high resistance alloy composed of 41.5% palladium, 50% gold, 5% molybdenum and 3.5%

lron.

6. Resistance material for precision resistance electrical apparatus or equipment, made of the alloy as claimed in claim 1.

7. Resistance material for. precision resistance e1ec trical apparatus or equipment made of the alloy claimed in c laimj4'.

References; Cited; in the. ile of this patent UNITED STATES PATENTS 2,001,017 Feussner etal May 14, 1935 2,780,543 Schneideret al. Feb. 5, 1957 FOREIGN PATENTS 454,019 Canada Q- Ian. 11, 1949 

1. A DUCTILE STABLE HIGH RESISTANCE ALLOY COMPOSED OF 16-90% PALLADIUM, 5-64% GOLD AND 2.5-20% MOLYBDENUM APART FROM IMPURITIES. 